1. Field of the Invention
The present invention relates to thin film transistors, and in particular relates to methods and structures of utilizing an oxide semiconductor as channel layers of the thin film transistors.
2. Description of the Related Art
In the process of forming thin film transistors (TFT), oxide semiconductors have become a main trend for related industries in Japan and Korea. The oxide semiconductors can be zinc oxide (ZnO), gallium-doped zinc oxide (GZO), aluminum zinc oxide (AZO), zinc tin oxide (ZTO), indium zinc oxide (IZO), and indium gallium zinc oxide (IGZO), and the likes. The oxide semiconductor process may adopt a five photomask process such as a back channel etching (BCE) process or a coplanar process for an inverted gate electrode, or adopt a six photomask process such as a process utilizing an etching stop layer. The etching stop layer in the process adopting six photomasks can protect a channel layer, such that devices made thereby have performances which are better than that of the process adopting five photomasks. However, the process utilizing the etching stop layer needs an additional photomask. In the coplanar process for inverted gate electrodes adopting five photomasks, each photomask can individually define each layer of the TFT without being limited by etching selectivity. As such, panel manufacturers only need to slightly change the processes, and benefiting mass production. In the future, large area panels will combine copper metal processes with the oxide semiconductor technology. Before forming a passivation layer covering a copper metal layer, the oxide on the metal surface should be reduced to copper by plasma of reducing atmosphere (e.g. H2 plasma). The oxide semiconductor is sensitive to the plasma of reducing atmosphere, and the reducing plasma may break a device made thereby.
Accordingly, a novel process without additional photomasks is called for, which efficiently protects the channel layer from damage of following processes, such as the reducing plasma process.